Communications interface for the serial transmission of digital data, and corresponding data transmission method

ABSTRACT

A communications interface and a data transmission method for the serial transmission of digital data are described, in which at least three signal lines (Tx 0 , Tx 1 , Tx 2 ) are provided, which can each have a “high” or a “low” level impressed on them, and a data bit which is to be transmitted can be coded by changing the level of two of the at least three signal lines, and hence as a result of the transition from a first level triplet and to a second level triplet.

BACKGROUND INFORMATION

With serial data transmission, it is often necessary to make acompromise between the parameters of speed, interference immunity andcurrent consumption. The highest data throughput at a given clockfrequency is achieved by synchronous transmission using separate data,clock and control lines. However, such synchronous transmission isparticularly susceptible to interference, and it has only conditionalsuitability for the use of data protection mechanisms for recognizingmultiple errors.

Other methods, such as synchronous transmission with clock recovery, orasynchronous transmission, are significantly slower at the same clockfrequency. The reason for this is the necessary transmission ofadditional information for synchronization and the multiple samplingrequired. The transmission rate can be increased by the proportionallyrising current consumption only to a limited extent.

SUMMARY

An object of the present invention is to provide a communicationsinterface for the serial transmission of digital data and a serial datatransmission method for the transmission of digital data on a bit by bitbasis, each of which can be used to achieve serial transmission which isimmune to interference and has the speed advantages of synchronoustransmission and reliable synchronization between the clock and thedata.

Conventionally the transmission path is designed by shielding orlimiting the length, to be so reliable that recognition of individualerrors is sufficient to ensure transmission with adequate interferenceimmunity. In this case, the start and end of data transmission aresynchronized using start/stop synchronization via the control line. Bitsynchronization is monitored by counting the clock pulses between startand stop. The data itself is protected by means of a parity bit.

In the case of synchronous transmission with clock recovery, the usefulinformation is coded such that the resultant bit stream contains anadequate number of edge changes (Manchester coding, bit stuffing, 4B/5Betc.), which can be used to recover the send clock signal (bitsynchronization). Start/stop synchronization is carried out usingspecial bit sequences which do not occur in the rest of the telegram(BOF, EOF). The disadvantage of this is the relatively large volume ofdata to be transmitted as a result of coding (factor 1.25 . . . 2). Inthis context, the additional start and end identifier is of particularconcern with small volumes of useful data.

With asynchronous transmission, bit synchronization is carried out usinga start and stop bit. However, synchronization is assured only during alimited number of bit times, so that this sequence has to be repeatedregularly. Start/stop synchronization of a telegram is carried out, asit is for synchronous transmission with clock recovery, with an explicitstart and end identifier. The disadvantage here, too, is the relativelylarge volume of data to be transmitted. In addition, with asynchronoustransmission, multiple sampling is necessary, which either reduces thedata rate or increases current consumption.

The present invention solves the problem in that the informationelements to be transmitted, the data, the clock signal and the start andend of data transmission (start/stop), are converted into special statesequences in the transmitter in accordance with a defined codingspecification, and are transformed back from these state sequences intothe information elements again in the receiver. To this end, acommunications interface for the serial transmission of digital data isprovided, in which at least three signal lines are provided which caneach have a “high” or a “low” level impressed on them, and a data item,which is to be transmitted, can be coded by changing the level of two ofthe at least three signal lines (Tx0, Tx1, Tx2), and hence as a resultof the transition from a first level n-tuple to a second level n-tuple.

In the case of exactly three signal lines (Tx0, Tx1, Tx2), the data itemto be transmitted can be coded as a result of the transition from afirst level triplet to a second level triplet. Since the communicationsinterface can also be used to transmit synchronization information, forexample, the term “data item” additionally at least includes suchinformation as well.

The permissible transitions from a first level n-tuple or first leveltriplet to a second level n-tuple or second level triplet are defined ina coding scheme and can be stored in this form in the communicationsubscribers, which can be communicatively connected via thecommunications interface. The coding schemes in the receiver cantherefore also be used, in particular, for error recognition andsuppression.

This makes it possible to carry out a serial data transmission methodfor the bitwise transmission of digital data on a bit by bit basis usingat least three signal lines which can each have a “high” or a “low”level impressed on them, a data bit which is to be transmitted beingcoded by changing the level of two of the at least three signal lines(Tx0, Tx1, Tx2), and hence as a result of the transition from a firstlevel triplet to a second level triplet.

In this case, data transmission is particularly immune to interferenceif the data bit which is to be transmitted is coded by a change in thelevel of two of the at least three signal lines.

If the change in the level of the at least two signal lines occurscontradirectionally, susceptibility to interference is reduced evenfurther.

If the start and end of data transmission can be coded by inverting therespective levels of the at least three signal lines, start/stopsynchronization with only one bit time is possible, so that, with shorttelegrams, a high data throughput is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principle of state sequence coding using a statediagram according to the example embodiment of the present invention

FIG. 2 shows the transmission of useful data with the state sequencecoding according to the example embodiment of the present intention.

DETAILED DESCRIPTION

In accordance with a defined coding specification, which is explainedbelow with reference to FIG. 1, the information elements to betransmitted are converted into special state sequences.

The state diagram shown in FIG. 1 has six nodes with transitions definedbetween these nodes. Each of the nodes represents a level, status orstate triplet. During bus transmission, only the states 001_(B), 010_(B)and 100_(B) are used, because, with these state triplets, exactly twolevels change contradirectionally whenever there is a transition from afirst to a second triplet. Under these circumstances, the transitionfrom the level triplet 001_(B) to the level triplet 010_(B) identifiesthe: transmission of a logic 1; the transition from the level triplet001_(B) to the level triplet 100_(B) identifies the transmission of alogic 0; etc.

During bus inactivity R, only the states 110_(B), 101_(B) and 011_(B)are used, since, again, exactly two levels change contradirectionallyduring transition from a first state triplet of this quantity to asecond state triplet of this quantity.

To change from the operating mode of data transmission T to theoperating mode of bus inactivity R or to change from the operating modeof bus inactivity R to the operating mode of data transmission T, astart/stop transition is necessary. This start/stop transition isdistinguished by inversion of all three levels. If, in the last step ofdata transmission, a logic 1 is transmitted with the change from thelevel triplet 010_(B) to the level triplet 100_(B), there follows a stoptransition from the level triplet 100_(B) to the level triplet 011_(B).This is followed by the transition to the operating mode of businactivity R, in which the levels associated with the level triplet110_(B) are applied to the three signal lines. From this state of businactivity R, a start transition is used to change to the operating modeof data transmission T—level triplet 001_(B)—again, a logic 1 beingidentified by a transition to the level triplet 010_(B) and a logic 0being identified by a transition to the level triplet 100_(B).

The present state of the three signal lines is denoted by a respectivenode on the state diagram. At a particular instant, only one of thenodes on the state diagram is ever valid. The nodes on the state diagramdescribe, as it were, positions on a path, the transitions definedbetween the nodes stipulating the possible paths. Transitions notdefined on the state diagram are not possible; thus, FIG. 1, directtransition from the level triplet 011_(B) to the level triplet 011_(B),for example, is not possible. The coding specification defined using thestate diagram shown in FIG. 1 is called a state sequence coding. Inaddition, it is also possible to use data protection mechanisms forrecognizing multiple errors and for error correction.

FIG. 2 shows an example of data transmission based on the principle ofstate sequence coding. Both the transmitter S (first ordinate in theupper third) and the receiver E (first ordinate in the lower third)contain the data D, D′ in bit-serial form. The data D, D′ isdistinguished conventionally by alternate “high” and “low” levels, aplurality of successive similar levels being distinguishable on thebasis of the clock signal C (central ordinate, both in the upper thirdand in the lower third). Data transmission starts when a high level isimpressed on the start/stop line and ends when the level of this linechanges to low again (third ordinate both in the upper and in the lowerthird).

The central third shows the signal sequence on the transmission path L,produced by the three signal lines (Tx0, Tx1, Tx2) after thetransmission data D has been subjected to state sequence coding (firstordinate, upper third). The transmission data D is converted intobit-serial data D′ again in the receiver E in accordance with the codingspecification of the state sequence coding (first ordinate, lowerthird).

What is claimed is:
 1. A communications interface for serialtransmission of digital data, comprising: at least three signal linesupon which a “high” signal or a “low” signal is impressed; and a dataitem encoded by changing a level of at least two of the at least threesignal lines to provide a transition from a first level n-tuple to asecond level n-tuple.
 2. The communications interface according to claim1, wherein the first level n-tuple and the second level n-tuple are afirst level triplet and a second level triplet, respectively.
 3. Thecommunications interface according to claim 1, wherein the level changeon the at least two signal lines occurs contradirectionally.
 4. Thecommunications interface according to claim 1, wherein a start of datatransmission and an end of data transmission is coded by invertingrespective levels of the at least three signal lines.
 5. A method ofserial transmission of data for transmitting digital data on abit-by-bit basis, comprising: providing at least three signal lines uponwhich a “high” signal or a “low” signal is impressed; and transmitting adata item by changing the level of two of the at least three signallines to provide a transition from a first level n-tuple to a secondlevel n-tuple.
 6. The method according to claim 5, wherein the firstlevel n-tuple and the second level n-tuple are a first level triplet anda second level triplet, respectively.
 7. The method according to claim5, further comprising: coding a start of data transmission and an end ofdata transmission by inverting respective levels of the at least threesignal lines.
 8. A programmable controller for at least one ofcontrolling and monitoring a technical process, the controller beingcommunicatively connected to other programmable controllers, comprising:a communications interface for serial transmission of digital data, theinterface including at least three signal lines upon which a “high”signal or a “low” signal is impressed, and a data item encoded bychanging a level of at least two of the at least three signal lines toprovide a transition from a first level n-tuple to a second leveln-tuple.
 9. A programmable controller for at least one of controllingand monitoring a technical process, the controller being communicativelyconnected to other programmable controllers, comprising: acommunications interface for serial transmission of digital dataconfigured to perform the steps of providing at least three signal linesupon which a “high” signal or a “low” signal is impressed, andtransmitting a data item by changing the level of two of the at leastthree signal lines to provide a transition from a first level n-tuple toa second level n-tuple.